Subsidence Characteristics of Hydrate-Bearing Sediments during Depressurization: Insights from Experimental and Discrete Element Method Simulations

Reservoir subsidence induced by natural gas hydrate (NGH) dissociation is a critical safety issue during gas production from NGH reservoirs. Concerning the limited field monitoring of reservoir subsidence, this study employs a customized apparatus to investigate the subsidence behaviors of methane-hydrate-bearing sediments, considering the effects of three reservoir factors, including hydrate saturation, skeleton type, and effective overburden stress. The experimental results show that the NGH reservoir subsidence process during depressurization is controlled by pore pressure reduction and hydrate dissociation; the former mainly affects the effective stress, and the latter mainly affects the mechanical properties of gas hydrate-bearing sediments (GHBSs). The dominance of the two factors on hydrate reservoir subsidence is a dynamic and competitive process during depressurization, and the impact of hydrate dissociation becomes significant in the high hydrate saturation case. Different from coarse-grained sediments, delayed subsidence and intermittent compaction are observed in fine-grained hydrate-bearing sediments, and they are controlled by the initial hydrate saturation and permeability of the skeleton. The evolution of GHBS subsidence is similar under various effective overburden stress and it increases with effective stress. Combined with numerical simulations based on the discrete element method, it is illustrated that the lateral displacement fixed boundary, radial inhomogeneous distribution of hydrate, and excessive sample height-to-diameter ratio may led to conservative sample subsidence in laboratory experiments compared to a field NGH reservoir.